System for measuring liquid flow rates

ABSTRACT

A system for monitoring non-volatile residue concentrations in ultra pure water includes a nebulizer for generating an aerosol composed of multiple water droplets, a heating element changing the aerosol to a suspension of residue particles, and a condensation particle counter to supersaturate the dried aerosol to cause droplet growth through condensation of a liquid onto the particles. The nebulizer incorporates a flow dividing structure that divides exiting waste water into a series of droplets. The droplets are counted to directly indicate a waste water flow rate and indirectly indicate an input flow rate of water supplied to the nebulizer. The condensation particle counter employs water as the condensing medium, avoiding the need for undesirable chemical formulations and enabling use of the ultra pure water itself as the condensing medium.

This application is a divisional of U.S. Non-Provisional patentapplication Ser. No. 11/935,810 entitled “System for MeasuringNon-Volatile Residue in Ultra Pure Water,” filed Nov. 6, 2007, now U.S.Pat. No. 7,777,868.

This application claims the benefit of priority based on ProvisionalApplication No. 60/857,548 entitled “System for Measuring Non-VolatileResidue in Ultra Pure Water” filed Nov. 7, 2006.

BACKGROUND OF THE INVENTION

The present invention relates to instruments for measuring minuteconcentrations of impurities in liquids, and more particularly tosystems for detecting non-volatile residue concentrations in ultra purewater and aerosol generating components used in such systems.

Certain industries, most notably semiconductor fabrication, involveextremely high standards of cleanliness and purity. A semiconductorcomponent may require washing with ultra pure water after eachprocessing stage, to remove chemicals used in that stage. Moregenerally, ultra pure water may be used to clean fixtures and othertools used to handle semiconductor wafers and other components. Anynon-volatile residue present in the ultra pure water can remain on thesurface of the component after the water has evaporated, possiblycausing defects in the resulting semiconductor device. Thus, there is aneed to monitor the ultra pure water used in such processes, to insurethat the concentration of non-volatile residue remains at or belowacceptable levels.

Systems have been developed and employed successfully to continuouslymonitor the quality of ultra pure water. U.S. Pat. No. 5,098,657(Blackford et al.) discloses a system in which ultra pure water isprovided at a constant flow rate to a nebulizer where the water isformed into droplets. The droplets are dried to provide non-volatileresidue particles. The particles can be detected electrostatically orprovided to a condensation particle counter (CPC, also known as acondensation nucleus counter) where the particles are “grown” intolarger droplets and sensed optically. Droplets are grown with a fluidhaving a relatively low vapor mass diffusivity, e.g. butyl alcohol.

While such systems have enjoyed success, they also are subject todifficulties that limit their utility. One of these concerns ismeasuring the flow rate of the ultra pure water to the nebulizer. Suchmeasurement is critical, because any change in the flow rate, e.g. dueto a blockage in the water delivery system, seriously disruptsmeasurement of the residue concentration. The conventional approach isto position a rotometer just upstream of the nebulizer. Rotometers arenot particularly well suited for measuring the extremely low flow ratesinvolved, typically about 1 milliliter per minute. They are expensive,in part due to the need for ultra-clean materials such as Teflon tominimize residue contamination that would adversely affect concentrationreadings.

Another problem is the accumulation of waste water in the nebulizer.Conventional devices have employed sponges to absorb waste water, butthis only postpones the eventual need to remove the waste water.

Another difficulty concerns the sapphire orifice plate typically used tocontrol the flow rate of water into the nebulizer. A forty microndiameter orifice through the plate limits the flow and admits water intothe nebulizer. The pressure drop across the orifice plate is sufficientto cause gasses dissolved in the water to accumulate on the back side ofthe orifice plate and form bubbles. Downstream of the nebulizer, thebubbles eventually break free and tend to disrupt residue concentrationmeasurements.

As to the condensation particle counter, a concern relates to the use ofbutyl alcohol or similar fluids with low vapor mass diffusivity forgrowing the residue particles into droplets. Such liquids tend to beflammable, toxic, and produce noxious odors that frequently requirevapor exhaust systems to be located near the measuring device.Frequently the liquids are subject to health and environmentalregulations that restrict their use in indoor environments. In addition,the liquids require equipment for supplying, collecting and draining theliquid involved.

Another persistent problem is the relatively long time elapsed between achange in the concentration of non-volatile residue in the ultra purewater, and the detection of the change. This raises the risk thatcontaminated water may be used in several process stages before thecondition is realized.

Accordingly, the present invention has several aspects directed to oneor more of the following objects:

-   -   to provide an aerosol generating device, e.g. a nebulizer, with        a reliable means for measuring an input flow rate of the liquid        supplied to the device without contacting or otherwise        interfering with the liquid used to generate the aerosol        droplets;    -   to provide an aerosol generating device particularly well-suited        for digital measurement of the flow rate of the liquid into and        through the device;    -   to provide a system and process for measuring concentrations of        non-volatile residue in test liquids, adapted to facilitate the        use of the test liquid as the condensation medium for droplet        growth onto previously dried particles for optical detection;        and    -   to provide a non-volatile residue measuring system with improved        response times for alerting users to changes in non-volatile        concentrations, liquid flow rates and other key parameters.

SUMMARY OF THE INVENTION

To achieve these and other objects, there is provided a device forgenerating an aerosol composed of multiple droplets of a test liquid.The device includes a first conduit for receiving a test liquid at aninput flow rate, and a second conduit for receiving pressurized gas. Amerger region, open to the first conduit and to the second conduit forsimultaneous reception of the test liquid and the pressurized air, isadapted to generate an aerosol composed of multiple droplets of a firstportion of the test liquid suspended in the gas. An aerosol exit passageis open to the merger region for conducting the aerosol away from themerger region. A liquid exit passage, open to the merger region, isadapted to conduct an output flow comprised of a second portion of thetest liquid away from the merger region. A flow sensor is disposed alongthe liquid exit passage and adapted to generate a sensor signalindicating an output flow rate of the output flow.

According to this aspect of the invention, the flow rate of ultra purewater into a nebulizer can be determined by measuring the rate at whichthe nebulizer outputs waste liquid. Measurement occurs at a pointdownstream of the nebulizer entrance, and is accomplished withoutcontact or other interaction with the nebulizer droplets used to measureresidue concentration. Thus, the flow rate measuring component has noimpact on residue concentration readings. This approach does not takeinto account the entire flow of the ultra pure water or other testliquid, although nearly 95 percent of the incoming water becomes wastewater. The specific percentage of incoming water that becomes wastewater can vary from one nebulizer to another, yet the percentage withinany given nebulizer is constant. Consequently, the system can becalibrated to determine the incoming flow rate based on the waste wateroutput flow rate.

Preferably, the first conduit comprises an elongate axially directedflow restricting opening or bore adapted to gradually reduce a pressureof the test liquid in the downstream direction toward the merger region,so that the test liquid enters the merger region at a pressure justabove atmospheric pressure. For example, the first conduit can take theform of an extended length of microbore tubing with a restricted (e.g.500 microns in diameter) axial passage.

The microbore tubing, like the previously used orifice plate, controlsthe flow rate of the water into the nebulizer, with the added benefit offorming a gradual reduction in water pressure, to just above atmosphericpressure at the nebulizer entrance. With the severe pressure drop at theentrance avoided, gasses dissolved in the ultra pure water tend toremain in solution rather than form gas bubbles. Consequently,downstream disruptions in residue measurements due to gas bubbles areminimized or completely avoided.

Another aspect of the present invention is a droplet generating and flowmeasuring apparatus. The apparatus includes an input conduit defining anupstream region of a liquid path for conveying a test liquid at an inputflow rate. A merger region is open to the input conduit to receive thetest liquid for merger with pressurized gas, to generate an aerosolcomposed of multiple droplets of a first portion of the test liquidsuspended in the gas. An aerosol exit passage is open to the mergerregion for conveying the aerosol away from the merger region. A liquidexit passage, defining a downstream region of the liquid path, is opento the merger region to convey a second portion of the test liquid awayfrom the merger region at an output flow rate. A flow dividing structureis disposed along the liquid path and adapted to separate the testliquid into discrete test liquid segments substantially uniform involume. A flow sensor is disposed at a sensing location along the liquidpath downstream of the flow dividing structure, to detect each of thetest liquid segments as it passes the sensing location.

The preferred flow dividing structure is a vessel disposed downstream ofthe merger region to collect the second portion of the test liquid. Thevessel has an orifice at its bottom adapted to serially release the testliquid segments in droplet form. For example, the aerosol generator canbe provided with a weir and standpipe arrangement. As water collects ina trap at the bottom of the aerosol generator, it rises above the weirand overflows into the standpipe. A nozzle at the bottom of thestandpipe forms the orifice, which is sized to prevent the water orother test liquid from continuously or rapidly draining. Instead, due tosurface tension effects, liquid is prevented from leaving the standpipeuntil the collected liquid reaches a threshold, whereupon the liquid hassufficient weight to overcome surface tension and exits the standpipe asa droplet. If the pressure in the standpipe remains substantiallyconstant, the size of the droplets likewise is constant, and the wastewater flow rate is determined by the frequency at which the dropletsleave the collection volume. Individual droplets are counted by opticalcomponents, to measure waste liquid flow rate and, through calibration,to determine the flow rate of liquid into the nebulizer.

The division of the waste liquid flow into individually counted dropletsis particularly well suited for generating digital data. Signalsgenerated when counting the droplets can be provided directly to adigital processor, with no need for an analog-to-digital converter. Anadditional benefit of this approach is that the droplets exit thenebulizer rather than accumulating within the nebulizer. At the sametime, water remains in the trap, weir and standpipe after each dropletleaves the standpipe. The remaining water acts as a seal to preventoutside air from entering the nebulizer. Outside air entry must beprevented, since airborne particles otherwise would mingle with theresidue agglomerate particles leaving the nebulizer and cause erroneousresidue concentration readings.

Another aspect of the invention is an instrument for measuringnon-volatile residue in a test liquid. The instrument includes a conduitarrangement comprising an entrance conduit for conveying a test liquiddownstream, and first and second downstream conduits fluid coupled tothe entrance conduit for respectively conveying first and secondportions of the test liquid. An aerosol generating stage is fluidcoupled to the first downstream conduit to receive the first portion ofthe test liquid, and is adapted to generate an aerosol comprised ofmultiple test liquid droplets suspended in a gas. An aerosol dryingstage is provided downstream of the aerosol generating stage forevaporating the test liquid as the aerosol is conveyed therealong,whereby the aerosol downstream of the drying stage consists essentiallyof non-volatile residue particles suspended in the gas. A dropletforming stage, downstream of the drying stage, includes a holdingcomponent adapted to receive a condensing medium in liquid form andrelease the condensing medium in vapor form as the aerosol is conveyedtherealong, to supersaturate the aerosol and cause droplet growththrough condensation of said medium onto the residue particles. Adroplet detector is disposed at a sensing location downstream of thedroplet forming stage and adapted to detect the droplets resulting fromthe condensation as they pass the sensing location. The droplet formingstage is coupled to the second downstream conduit to receive the secondportion of the test liquid, and to provide the second portion of thetest liquid to the holding component as the condensing medium.

In systems used to monitor ultra pure water, this entails the use ofwater as the working medium in the condensation particle counter. Usingwater avoids the health and environmental concerns associated with butylalcohol and other perfluorinated hydrocarbons. This eliminates the needto supply, store and recover such fluids, and to separate such fluidsfrom the ultra pure water. Further, since the ultra pure water beingmonitored serves as the working medium in the condensation particlecounter, the working medium can be supplied and replenished through adirect connection of the CPC to the water supply.

When water is used as the working medium, the aerosol stream issaturated with water vapor and proceeds to a condensing regionsurrounded by wetted walls that are heated to provide a temperaturehigher than that of the saturated aerosol stream. Maximumsupersaturation occurs at the center of the aerosol flow, because themass diffusivity of water exceeds the thermal diffusivity of air.

One of the advantages of using water as the working fluid in a CPC is asubstantially higher threshold at which spontaneous nucleation (alsocalled homogeneous nucleation) can occur compared with the previouslyavailable butyl alcohol based CPC. An improved coincidence correctionalgorithm in the water-based CPC also contributes to a considerablyhigher permitted particle/droplet throughput rate. Further, a muchshorter drying column and aerosol path from the nebulizer to the CPCconsiderably reduce diffusion losses and the time elapsed fromgenerating the aerosol to sensing droplets to generate residueconcentration information. As a result of these advantages, theconcentration information from the CPC is available virtually in realtime, and can encompass concentrations ranging from a single part pertrillion to 60 parts per billion in the single count mode. If desired, aphotometric mode can be employed to increase the upper limit to morethan 500 parts per billion.

Another aspect of the invention is a process for determining an inputflow rate of a liquid, provided to a nebulizer in a fluid measurementsystem incorporating the nebulizer and a processor coupled to thenebulizer to receive information from the nebulizer. The processincludes:

(a) providing a liquid to a nebulizer at least one known input flowrate;

(b) while so providing the liquid, using the nebulizer to generate anaerosol composed of multiple droplets of a first portion of the liquidsuspended in a gas, while conveying a second portion of the liquid alongan exit passage of the nebulizer;

(c) measuring a flow rate of the second portion of the liquid todetermine an output flow rate corresponding to the at least one knowninput flow rate;

(d) providing the known input flow rate and the corresponding outputflow rate to a processor coupled to the nebulizer; and

(e) associating the corresponding output flow rate with the known inputflow rate within the processor to enable the processor to generate anindication of the known input flow rate in response to receiving anindication of the corresponding output flow rate from the nebulizer.

The input and output rates may be associated through a calibrationprocess in which several different input rates lead to the measurementof several different output flow rates corresponding individually to theinput rates. The rates can be stored to the processor as a look-up tablewhereby the processor, upon receiving an indication of a measured outputflow rate, is caused to generate the corresponding input flow rate.

Alternatively, the input and output flow rates may be associated througha function, e.g. a direct linear function based on a determination thatfor the nebulizer involved, the output flow rate is a certain percent ofthe input flow rate.

Yet another aspect of the invention is a process for measuringnon-volatile residue concentrations in a monitoring system including anaerosol forming stage and a droplet growth stage downstream of theaerosol forming stage. The process includes:

(a) providing a test liquid flow to a non-volatile residue monitoringsystem;

(b) using a first portion of the test liquid flow to generate an aerosolcomposed of multiple droplets of the test liquid suspended in a gas;

(c) drying the aerosol to evaporate the test liquid and thereby providea dried aerosol consisting essentially of multiple non-volatile residueparticles suspended in the gas;

(d) using a second portion of the test liquid flow to supersaturate thedried aerosol and cause droplet growth through condensation of the testliquid onto the non-volatile residue particles; and

(e) following the droplet growth, detecting the droplets formed by saidcondensation to determine a concentration of the non-volatile residue inthe test liquid.

Thus, non-volatile residue measuring systems configured according to thepresent invention generate more reliable concentration information invirtually real time and over a wider range of residue concentrations.The critical flow rate of ultra pure water to the nebulizer is monitoredwithout contacting the ultra pure water used to provide the nebulizeroutput, and in a manner particularly well suited to generating digitalflow rate and concentration data.

IN THE DRAWINGS

For a further understanding of the foregoing features and advantages,reference is made to the following detailed description and to thedrawings, in which:

FIG. 1 is a diagrammatic view of a non-volatile residue measuring systemconfigured according to the present invention;

FIG. 2 is a more detailed schematic view of part of the system;

FIGS. 3-5 are sectional views illustrating a nebulizer of the system;

FIGS. 6-9 schematically illustrate the operation of a flow measurementcomponent of the nebulizer; and

FIG. 10 is a schematic view of a condensation particle counter of thesystem.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning to the drawings, FIG. 1 is a diagram of a system 16 formonitoring the concentration of non-volatile residue in ultra purewater. The water may be used in a semiconductor fabrication processstage or other application requiring high purity. System 16 continuouslymonitors the water to insure an acceptably low residue concentration asthe water is supplied to the process stage.

As seen in the figure, ultra pure water from a water supply 18 and gasfrom a compressed air or nitrogen source 20 are supplied to a nebulizer22 to generate an aerosol including droplets of the water suspended inthe air or nitrogen. The aerosol is provided to a condensation particlecounter (CPC) 24. Most of the water provided to nebulizer 22, typicallyclose to 95 percent, is not used to form droplets, but instead isprovided to a flow sensor 26 used to monitor the flow rate of water intothe nebulizer through direct measurement of the waste water flow. Thewaste water is drained from the nebulizer after flow sensing, asindicated at 28.

The aerosol output of nebulizer 22 is dried to reduce the aerosol tosuspended residue particles provided to particle counter 24. As theaerosol travels through the particle counter, it is first saturated, andthen channeled through a condensation or supersaturation region in whichthe residue particles act as nuclei for water condensation. Thus, theresidue particles “grow” into considerably larger droplets that areoptically detected and counted to generate non-volatile residueconcentration information. The concentration information is provided toa microprocessor 30. The microprocessor provides the information to avideo display terminal 32 to generate a continuously updated record ofnon-volatile residue concentration in the ultra pure water. Themicroprocessor also receives water flow rate information from flowsensor 26.

Particle counter 24 includes an exit 33 through which the aerosol isdrawn out of the CPC after the droplets are counted. Water used in theCPC saturation and condensation regions is provided from water supply 18via a line 34.

FIG. 2 illustrates system 16 in more detail. Ultra pure water fromsupply 18 is received through a sapphire flow-limiting orifice 36 anddelivered to a water pressure regulator 38. Compressed air source 20provides air under pressure to an air pressure control regulator 40. Theair pressure control regulator controls air pressure to the waterpressure regulator 38. An air pressure of 30 psi produces a waterpressure of 30 psi. A pressure transducer 41 is operably coupled towater pressure regulator 38 to insure maintenance of the desiredpressure. The water pressure regulator automatically shuts off the flowof water to downstream system components, which eliminates the potentialfor water leaks that might occur if the water continued to flow after aloss of the air pressure.

The ultra pure water proceeds through a porous (20-60 micron) sinteredsteel filter 42 designed to remove coarse material such as plasticfragments, which otherwise might block the flow at downstreamcomponents. A 3-way tee 44, downstream of filter 42, incorporates acontrol orifice having a diameter of 430 micrometers which restricts thewater flow to the tee to about 100 milliliters per minute at 30 psiwater pressure. Tee 44 divides the ultra pure water flow into a sampleflow of about 1 milliliter per minute, and a waste water flow of about99 milliliters per minute.

A capillary 46 guides the sample flow from tee 44 to an entrance 48 ofnebulizer 22. Capillary 46, preferably a section ofpolytetrafluoroethylene (PTFE) or perfluoroalkoxy (PFA) tubing with a500 micron axial bore, controls the flow rate of water into thenebulizer, and produces a pressure gradient that reduces the waterpressure to just above atmospheric pressure at nebulizer entrance 48.

The water pressure along capillary 46 is reduced gradually andcontinually over the capillary length in the downstream direction. Thus,for a given capillary inlet water pressure, a longer capillary providesa liquid flow at a lower pressure at the capillary exit (i.e. thenebulizer entrance). In system 16, the preferred length of the capillaryis about 9 inches (23 cm). The optimum capillary length can depend on avariety of factors, including water pressure at tee 44, pressure of airentering nebulizer 22, and the exact diameter of the capillarymicrobore.

In a general sense, capillary 46 provides a flow-controlling conduit tothe nebulizer in which the liquid pressure upstream of the nebulizer isreduced, at a rate of at most about one pound per square inch, over eachinch of capillary length. More preferably, each inch of capillary lengthentails a pressure reduction in the range of 0.5-1.0 psi. For example,capillary 46 may reduce the pressure from about 5-10 psi at a point justbeyond filter 44 to about 1 psi at the nebulizer entrance. In thepreferred approach, the capillary has a uniform diameter axial bore overits complete length, and the desired rate of pressure reduction isachieved over the full capillary length.

With the gradual decrease in water pressure along capillary 46, thetendency for gasses dissolved in the water to form bubbles issubstantially eliminated. This avoids a problem in previous residuemonitoring systems, in which gas bubbles forming at the nebulizerentrance and then passing through the nebulizer would momentarilydisrupt downstream residue measurement. To further reduce this problem,tee 44 is advantageously oriented to direct the sample stream downwardwhile directing the waste water stream upward, whereby any bubblespresent in the water tend to rise with the waste stream.

Air from source 20 is provided at about fifty psi to a pressureregulator 50. Downstream, the air passes through a high efficiencyparticle air (HEPA) filter 52, and then is supplied to a nebulizer airentrance 54 at a pressure of 15 psi and a flow rate of 0.55 liters perminute through a conduit 56. Further, air is provided to a pointdownstream of nebulizer 22 through a conduit 58. Conduit 58 includes acontrol orifice 60 for limiting the air flow to a rate of about 2 litersper minute.

FIGS. 3, 4 and 5 illustrate nebulizer 22 in greater detail. Nebulizer 22includes a housing section 62 forming a merger region 64 where incomingwater and pressurized air mingle to form multiple droplets. The flow ofpressurized air into the nebulizer at 54 creates a slightly negativepressure in region 64 that draws water into the nebulizer from capillary46. The droplets leave nebulizer 22 as an aerosol, traveling upwardlythrough a passage 66. The proportion of residue relative to water varieswith the purity of the water, but is substantially constant overdifferent droplet sizes. Nebulizer 22 generates droplets of differentsizes, but there is sufficient consistency in the aggregate such thatthe droplet count, or more accurately the count of residue particlescorresponding to the droplets, is a reliable indicator of non-volatileresidue concentration in the water.

A thermoelectric device 68 functions as a heat sink to maintain a stabletemperature of about 25° C. in the nebulizer merger region. Thispromotes more uniform droplet sizes. As the aerosol leaves the nebulizervia aerosol passage 66, a heating element 70 along passage 66 evaporatesthe water to transform the aerosol into a suspension of residueparticles.

A housing section 72 below housing section 62 forms a waste waterreceiving compartment 74, a downstream holding compartment 76, and afluid passage 78 located between and coupling compartments 74 and 76.Water not forming the aerosol, i.e. a major portion (e.g. 95 percent) ofthe water entering nebulizer 22, descends directly into compartment 74,where the water accumulates as indicated at 82.

A water retention vessel 84 includes an upright cylindrical standpipe 86centered within holding compartment 76, and a truncated conical wall 88that converges downwardly at about a 60 degree angle to an exit orifice90.

An upper region of holding compartment 76 is open to atmosphericpressure through a passage 92. A cylindrical interior wall 94 of theholding compartment cooperates with standpipe 86 to form an annularwater holding region 96. A plug 98 is removable from housing section 72to drain the holding region. The hole for plug 98 is also used formachining

Waste water at first accumulates in receiving compartment 74, thenproceeds through passage 78 to holding region 96. The pressurized airflowing into the nebulizer creates a positive pressure in the receivingcompartment. As a result, water in holding region 96 is pushed upwardlyuntil the water level reaches the top of standpipe 86. Further additionof water in receiving compartment 74 causes the water to spill overstandpipe 86 into vessel 84. The standpipe thus functions as a weir,with water pushed upwardly and over the top of the weir due to thepositive pressure in the receiving compartment.

So long as the levels of water in receiving compartment 74 and holdingregion 96 are above passage 78, the passage functions as a trap toisolate the interior of vessel 84 from the nebulizer interior, in thesense that the water prevents the direct passage of air or any other gasfrom one of these regions to the other.

This has several beneficial effects. First, it tends to isolate thevessel interior from any pressure fluctuations in the merger region orelsewhere within the nebulizer. Consequently, the pressure inside vessel84 is determined by ambient pressure outside of nebulizer 22. Thepressure is essentially constant, virtually unaffected by pressurefluctuations inside the nebulizer.

Another benefit is that while the interior of vessel 84 is exposed toambient air for pressure control, water in holding region 96, passage 78and receiving compartment 74 prevents ambient air from reaching thenebulizer interior.

Nebulizer 22 includes a fluid flow measuring section 100 disposed belowhousing section 72 and forming an open region below holding compartment76. Water leaving vessel 84 through exit orifice 90 descends through theopen region to a basin 102, from which the waste water is removed fromthe nebulizer.

A light emitting diode 104 and a detector 106 are mounted within housingsection 100 along the open region. The housing section further includesa reflective surface 108 exposed to the open region and positioned withrespect to the diode and sensor such that water descending through theopen region passes between the diode and the reflective surface. In thepreferred version, this arrangement affords convenient access to thediode and detector. As an alternative, a diode and detector can beplaced on opposite sides of the droplet region.

A feature of nebulizer 22 is that waste water is drained through exitorifice 90, eliminating the need to periodically extract waste waterfrom the nebulizer. Further, the waste water is released incrementally,in a manner especially conducive to digital measurement of fluid flow.

The manner of waste water flow measurement is perhaps best understoodfrom FIGS. 6-9, which schematically show conical wall 88 of vessel 84,diode 104, detector 106 and reflective surface 108. Exit orifice 90 hasa diameter (e.g. about 1 mm) selected for temporary retention of waterwithin vessel 84. More particularly, as seen in FIG. 6, water is presentin the vessel up to a lower threshold 110. The water is retained bysurface tension forces, which at this point overcome the tendency of thewater to pass through exit orifice 90 due to gravity.

In FIG. 7, water has accumulated to a level above the lower threshold.Water is suspended below the exit orifice, in an early stage of dropletformation.

In FIG. 8, water has accumulated to the point of reaching an upperthreshold 112. At this point, the weight of the water is sufficient toovercome surface tension effects. A droplet 114 is formed and breaksfree from the water remaining in the holding component (FIG. 9). Whenthe droplet breaks free, the water level descends to or near to thelower threshold, and the process repeats.

Water overflowing standpipe 86 tends to enter vessel 84 in bursts,intermittently overcoming surface tension to break the meniscus at thetop of the standpipe. To modulate these bursts and ensure a more uniformflow of droplets through orifice 90, a piece of felt or natural spongeis inserted into the standpipe.

It has been found that if the positive pressure in vessel 84 isessentially constant (i.e. subject only to changes in atmosphericpressure), the droplets leaving exit orifice 90 are substantiallyidentical in size. As a result, the speed or frequency of dropletformation depends on the rate at which waste water is supplied to thevessel. Thus, the droplet frequency provides a direct measurement of thewaste water flow rate.

The dimensions of machined parts and the proportion of incoming water towaste water can vary from one nebulizer to another. Accordingly,nebulizer 22 is calibrated so that the droplet frequency directlyindicates the flow rate of ultra pure water into the nebulizer.

Calibration involves supplying water or another liquid to nebulizer 22at a known constant input flow rate while operating the nebulizer togenerate an aerosol. In this manner a portion of the incoming liquid isused to faun the aerosol droplets, while a second portion or remainderof the liquid descends to receiving compartment 74, eventually to exitthe nebulizer in the form of droplets 114.

As each droplet 114 descends between diode 104 and reflective surface108, it momentarily alters the light received by detector 106. Inresponse, the detector provides an electrical pulse or signal toprocessor 30 via a line 116 (FIG. 1). As noted above, a substantiallyconstant pressure in vessel 84 results in a substantially uniform sizein droplets 114. The pulses are counted, and the frequency indicates thewaste water flow rate, i.e. an output flow rate corresponding to theknown input flow rate.

Calibration entails modifying processor 30 to individually associate aplurality of different output flow rates with a plurality of input flowrates. This can be accomplished by measuring multiple output rates inconjunction with supplying water to the nebulizer at multiple differentknown input flow rates, to create a look-up table in the processor.Alternatively, based on measuring a single output flow rate for a knowninput flow rate, or measuring several different output flow rates inconjunction with supplying water at several known flow rates, the usergenerates a function relating the input and output flow rates. Thefunction may be a direct linear function, e.g. that the output rate is agiven fraction or percent of the input flow rate. Or, the function maybe more complex. In either event, processor 30 is modified toincorporate the function, and to use the function to generate anindication of the input flow rate responsive to receiving an indicationof the output flow rate from the nebulizer.

To this end, and with reference to FIG. 1, processor 30 has a counterregister 35 for accumulating a count of pulses from detector 106, and aconversion register 37 in the form of a function or look-up table fordetermining the input flow rates. Thus, processor 30, based on storedcalibration data, generates the fluid flow rate based on the dropletcount.

The droplet generation frequency provides an accurate flow ratemeasurement, despite any fluctuation in the positive pressure inreceiving compartment 74. Vessel wall 86 tends to dampen any changes inflow rate due to differences in the positive pressure, smoothing theflow out of exit orifice 90. If desired, the droplet frequency can beused as feedback to adjust the flow rate.

Further, in spite of fluctuations in the positive pressure incompartment 74 or the flow rate into the nebulizer, the water level incompartment 74 prevents ambient air from entering the nebulizer. Waterin holding region 96 prevents ambient air from reaching the nebulizerinterior via passage 92. Ambient air is kept out of the nebulizer toinsure that it cannot mingle with the aerosol, to prevent particlessuspended in the air from affecting residue concentration measurement.

As the aerosol stream proceeds upwardly in passage 66, it is heated byheating element 70 to a temperature of 120 degrees to evaporate theultra pure water, thus transforming the aerosol into a particlesuspension rather than a droplet suspension. A thermistor 111 (FIG. 4)monitors the temperature of the air steam as it exits passage 66. Thethermistor is used in a control loop to keep the temperature at 120degrees C. A thermal switch 113 is used to switch off the heater in theevent of overheating. Filtered dilution air flowing at a rate of about2.0 liters per minute enters through a port 115 to lower the dew point.The dried, diluted residue aerosol at a flow rate of about 2.5 litersper minute exits through a fitting 117 and is delivered to the dropletgrowth stage, condensation particle counter 24.

Returning to FIG. 2, the aerosol path from nebulizer 22 to particlecounter 24 is about 18 centimeters in length, as compared to theapproximately 80 centimeter path in conventional systems. The shorteraerosol path considerably reduces particle loss due to diffusion. Theshorter water pathway coupled with the shorter aerosol pathway alsoconsiderably reduces the system response time, to about 90 seconds ascompared to nearly ten minutes in conventional systems. Thus,unacceptable concentrations of impurities in the water are determinedearlier, and damage can be minimized through more immediate correctiveaction.

FIG. 10 illustrates condensation particle counter 24 in more detail. TheCPC includes a droplet growth column 118 including a substantially rigidcylindrical outer wall 120 and a porous cylindrical inner liner or wick122. Wick 122, formed of a ceramic material, is adapted to receive andhold water, and thereby provide water vapor to an internal passage 124surrounded by the wick. If desired, wick 122 can be mounted removably tofacilitate inspection and convenient replacement. A lower, saturationregion 126 of passage 124 is maintained at a near ambient temperature,e.g. at 20° C. A heating element 128 is used to maintain an upper,droplet growth region 130 of the chamber at an elevated temperature,e.g. 60° C. As aerosol from nebulizer 22 proceeds upwardly throughpassage 124, it becomes saturated along region 126. As the aerosoltravels through region 130, it becomes supersaturated with water vapor.All particles in the aerosol having at least a threshold size of 5 nmbecome nucleation sites for droplet growth due to water condensation.

As the particles proceed upwardly through growth region 130, twocounteracting phenomena are at work. First, due to the elevatedtemperature the wetted wick generates increased water vapor, whichtravels radially inward away from the wick toward the center of passage124. Second, the heated walls tend to transfer heat to the dropletgrowth region. However, because of the relatively high mass diffusivityof water vapor, the water vapor reaches the center of passage 124 morequickly then the heat. Consequently the particles and their immediatelyadjacent air, even while being warmed, remain sufficiently cool forsupersaturation and the resulting condensation and droplet growth.

A laser diode 132 and light sensor 134 are disposed above droplet growthcolumn 118 on opposite sides of the aerosol stream. Each droplet altersor interrupts light transmission to the sensor to generate an electricalpulse. The pulses are provided to processor 30, and the pulse countyields the non-volatile residue concentration.

With reference to FIG. 2 as well as FIG. 10, a pump 136 draws theaerosol out of CPC 24 and provides it to a waste outlet 138.

Given the use of water rather than butyl alcohol as the condensationmedium, there is no need for equipment designed to supply, circulate andcollect the medium and maintain that medium separately from the ultrapure water. Further, the water being monitored can be used as the CPCcondensation medium. To this end, as seen in FIG. 2, water from tee 44not provided to the nebulizer is fluid coupled over a line segment 140through a backflush valve 142, a line segment 144, a solenoid valve 146and a line segment 148 to the CPC. In FIG. 1 this is represented by line34. As seen in FIG. 10, CPC 24 includes a reservoir 150 fluid coupled tothe water supply through solenoid valve 146. The solenoid valve normallyis closed. When a level sensor 152 in the reservoir senses that thewater level in the reservoir has receded below a predeterminedthreshold, it opens valve 146 to replenish the water supply in thereservoir. Reservoir 150 can be provided with a fitting for drainingexcess water, as indicated at 154.

Gravity and capillary forces move water from reservoir 150 to wick 122,to insure that the wick remains wetted to provide water vapor along thesaturation and growth sections.

Returning to FIG. 2, backflush valve 142 can be closed, and ultra purewater provided in a reverse direction along line 140 from a backflushconnection 158. The flow proceeds in the reverse direction through valve44, filter 42, regulator 38 and inlet orifice 36, to maintain systemefficiency by dislodging any blockage that might occur along thesecomponents. Compressed air from source 20 not provided to the nebulizeris directed through a conduit 160 and a venturi regulator 162 to aventuri flow guide 164. The air, along with waste water from line 140and nebulizer 22, is channeled by the venturi guide to waste outlet 138.

Thus, in accordance with the present invention, a system for monitoringnon-volatile residue concentrations in ultra pure water generates morereliable information virtually in real time, to facilitate moreeffective management of processes that depend on water purity. The flowrate of the ultra pure water is measured accurately without contactingwater used to generate the sample measurement aerosol, in a manner wellsuited for generating digital flow measurement data.

1. A device for generating an aerosol composed of multiple droplets of atest liquid, including: a first conduit for receiving a test liquid atan input flow rate; a second conduit for receiving pressurized gas; amerger region, open to the first conduit and to the second conduit forsimultaneous reception of the test liquid and the pressurized air,adapted to generate an aerosol composed of multiple droplets of a firstportion of the test liquid suspended in the gas; an aerosol exit passageopen to the merger region for conducting the aerosol away from themerger region; a liquid exit passage open to the merger region and ameasuring section downstream of the liquid exit passage, adapted toconduct an output flow comprised of a second portion of the test liquidaway from the merger region; and a flow sensor disposed along themeasuring section and adapted to generate a sensor signal indicating anoutput flow rate of the output flow.
 2. The device of claim 1 wherein:the liquid exit passage incorporates a flow dividing structure adaptedto separate the output flow into discrete test liquid segments; and theflow sensor is disposed at a sensing location downstream of the flowdividing structure to detect each test liquid segment as it passes thesensing location.
 3. The device of claim 2 wherein: the flow sensor isadapted to optically detect the test liquid segments and to generate anelectrical pulse responsive to each test liquid segment passing thesensing location.
 4. The device of claim 2 wherein: the flow dividingstructure comprises a vessel disposed to collect the output flow, thevessel having an orifice at a bottom thereof adapted to serially releasethe test liquid segments in droplet form.
 5. The device of claim 2wherein: the flow dividing structure further is adapted to separate theoutput flow into test liquid segments substantially uniform in volume.6. The device of claim 1 wherein: the liquid exit passage comprises afirst compartment downstream of the merger region for collecting theoutput flow, a second compartment downstream of the first compartment,and a wall between the first and second compartments adapted to directthe output flow from the first compartment into the second compartmentvia spillage over a top of the wall after the first compartment isfilled with the test liquid; and the second compartment is exposed toambient pressure, and is substantially isolated from the merger regionby the test liquid occupying the first compartment.
 7. The device ofclaim 6 wherein: the second compartment incorporates structure forreleasing the test liquid therefrom in discrete test liquid segments. 8.The device of claim 1 further including: an aerosol drying stagedisposed along the aerosol exit passage for evaporating the firstportion of the test liquid, whereby the aerosol downstream of the dryingstage is characterized by residue particles suspended in the gas; and aconcentration indicating component downstream of the drying stage andadapted to generate residue concentration information based on receivedresidue particles.
 9. The device of claim 1 wherein: the first conduitcomprises an elongate axially directed flow restricting orifice adaptedto gradually reduce a pressure of the test liquid in the downstreamdirection toward the merger region, whereby the test liquid enters themerger region at a pressure just above atmospheric pressure.
 10. Thedevice of claim 1 further including: a processor coupled to receive thesensor signal and adapted to generate an indication of the input flowrate based on the sensor signal.
 11. A droplet generating and flowmeasuring apparatus, including: an input conduit defining an upstreamregion of a liquid path for conveying a test liquid at an input flowrate; a merger region open to the input conduit to receive the testliquid for merger with pressurized gas to generate an aerosol composedof multiple droplets of a first portion of the test liquid suspended inthe gas; an aerosol exit passage open to the merger region for conveyingthe aerosol away from the merger region; a liquid exit passage defininga downstream region of the liquid path, open to the merger region toconvey a second portion of the test liquid away from the merger regionat an output flow rate; a flow dividing structure disposed along theliquid path and adapted to separate the test liquid into discrete testliquid segments; and a flow sensor, disposed at a sensing location alongthe liquid path downstream of the flow dividing structure to detect eachof the test liquid segments as it passes the sensing location.
 12. Theapparatus of claim 11 wherein: the flow sensor is disposed along thedownstream region of the liquid path.
 13. The apparatus of claim 12wherein: the liquid exit passage comprises a first compartmentdownstream of the merger region for collecting the second portion of thetest liquid, a second compartment downstream of the first compartment,and a wall between the first and second compartments adapted to directthe second portion from the first compartment into the secondcompartment by spillage over a top of the wall after the firstcompartment is filled with the test liquid; and the second compartmentis exposed to ambient pressure, and is substantially isolated from themerger region by the test liquid occupying the first compartment. 14.The apparatus of claim 11 wherein: the flow sensor is adapted tooptically detect the test liquid segments and to generate an electricalpulse responsive to each test liquid segment passing the sensinglocation.
 15. The apparatus of claim 11 wherein: the flow dividingstructure comprises a vessel disposed downstream of the merger region tocollect the second portion of the test liquid and having an orifice at abottom thereof adapted to serially release the test liquid segments indroplet form.
 16. The apparatus of claim 15 wherein: the secondcompartment incorporates the flow dividing structure.
 17. The apparatusof claim 11 further including: a processor coupled to the flow sensorand adapted to generate an indication of the input flow rate based on afrequency at which the flow sensor detects the test liquid segments. 18.The apparatus of claim 11 wherein: the flow dividing structure isadapted to separate the test liquid into segments substantially uniformin volume.
 19. In a fluid measurement system incorporating a nebulizerand a processor coupled to the nebulizer to receive information from thenebulizer, a process for determining an input flow rate of a liquidprovided to the nebulizer, including: providing a liquid to a nebulizerat least one known input flow rate; while so providing the liquid, usingthe nebulizer to generate an aerosol composed of multiple droplets of afirst portion of the liquid suspended in a gas, while conveying a secondportion of the liquid along an exit passage of the nebulizer; measuringa flow rate of the second portion of the liquid to determine an outputflow rate corresponding to the at least one known input flow rate;providing the known input flow rate and the corresponding output flowrate to a processor coupled to the nebulizer; and associating thecorresponding output flow rate with the known input flow rate within theprocessor to enable the processor to generate an indication of the knowninput flow rate in response to receiving an indication of thecorresponding output flow rate from the nebulizer.
 20. The process ofclaim 19 wherein: providing the liquid comprises providing the liquid tothe nebulizer at a plurality of different known input flow rates;measuring a flow rate comprises measuring a plurality of differentoutput flow rates, each corresponding to a different one of the knowninput flow rates; providing the known input flow rate and thecorresponding output flow rate comprises storing the plurality of knowninput flow rates and their corresponding output flow rates to theprocessor; and associating the flow rates comprises causing theprocessor to generate an indication of each one of the known input flowrates in response to receiving an indication of its corresponding outputflow rate from the nebulizer.
 21. The process of claim 19 wherein:associating the flow rates comprises determining a function relating theinput flow rate and the corresponding output flow rate, and modifyingthe processor according the function to configure the computer togenerate a plurality of different input flow rates, individually, inresponse to receiving indicia of a plurality of different correspondingoutput flow rates from the nebulizer.
 22. A liquid flow measuringapparatus, including: structure forming a holding region disposed alonga liquid flow path for accumulating liquid received from an upstreamregion of the liquid flow path; a vessel disposed downstream of theholding region in fluid communication with the holding region through aninterface allowing liquid to flow from the holding region into thevessel responsive to an accumulation of liquid in the holding regionexceeding a predetermined threshold, wherein liquid occupying theholding region substantially isolates the vessel from any pressurefluctuations along the upstream region of the liquid flow, the vesselhaving an orifice adapted to separate liquid exiting the vessel intodiscrete liquid segments; and a flow sensor disposed along the liquidpath at a sensing location downstream of the vessel to detect each ofthe liquid segments passing the sensing location.
 23. The apparatus ofclaim 22 wherein: the vessel is exposed to ambient pressure.
 24. Theapparatus of claim 22 wherein: the orifice is disposed at a bottom ofthe vessel to serially release the liquid segments as dropletssubstantially uniform in volume.
 25. The apparatus of claim 22 wherein:the vessel comprises an upright vessel wall, and the holding regioncomprises a compartment adjacent the vessel adapted to accumulate liquidto a level coinciding with a top of the vessel wall to determine saidthreshold whereby liquid exceeding the threshold enters the vessel byspillage over the wall.
 26. The apparatus of claim 22 further including:a processor coupled to the flow sensor and adapted to indicate a flowrate of liquid along the liquid flow path based on a frequency at whichthe flow sensor detects the liquid segments, wherein the flow sensor isadapted to optically detect the liquid segments and generate anelectrical pulse responsive to each liquid segment detected.
 27. Theapparatus of claim 22 further including: a housing forming a mergerregion disposed along the upstream region of the liquid path to receivethe liquid for merger with pressurized gas to generate an aerosolcomposed of multiple droplets of a first portion of the liquid suspendedin the gas, an aerosol exit passage open to the merger region forconveying the aerosol away from the merger region, and a liquid exitpassage open to the merger region to convey a second portion of theliquid from the merger region to the holding region.